Interference eliminator



Dec. 2?, 1938.

F. E. NICKEL INTERFERENCE ELIMINATOR Filed Oct. 16, 1936 2 Sheets-Sheetl INVENTOR ATTORNEYE muzmumm uzEommhuI VQLTH G E i INTERFERENCEELIMINATOR Filed Oct. 16, 1936 2 sheets-5mm? F FM; QUE may Z: L4 (5 I/M/ k 11 it. v:

j w, J'v Pi h A I; e 1 J L 0 m0 4m 9% 1200 1150a 18 g 2100 N v E i-tflEnrx'GI 71' 1 A. a 1 I .2 wil Patented Dec. 27, 1938 UNITED STATESPATENT OFFICE INTERFERENCE ELIMINATOR Application October 16, 1936,Serial No. 105,977

7 Claims.

This invention relates to electrical communication, particularly radiotelegraph communication, in which extraneous currents resulting fromvarious causes are superimposed on desired 5 signal currents, maskingthe latter and often making it impossible to intelligibly receive thesignal currents.

A broad object of the invention is to provide a practicable filteringsystem capable of passing l persisting electric current of desiredfrequency while attenuating currents of other frequencies to a markeddegree.

Another object is to provide a filtering system capable of passingpersisting currents of a desired 15 frequency that is relativelyinefficient in the production of currents of such frequency when shockexcited.

Various more specific objects and features of the invention will becomeapparent from the de- 20 tailed description of one embodiment of theinvention as employed in a C. W. (continuous wave) radio telegraphreceiving system.

As is well known, radio interference results from natural static andalso from local disturb- 25 ances caused by electricalmachinery, powercircuits, etc., all of which produce in the output circuit of the radioreceiver noise currents of various frequencies, most of which lie inthat portion of the audio frequency band to which the 30 ear is highlysensitive. Attempts have been made to reduce interference in C. W.telegraphy by making the signal note of relatively high frequency andtuning the audio circuit of the re ceiver sharply to the signalfrequency whereby 35 noise currents of frequencies other than the sig-.nal frequency are discriminated against. The results are less favorablethan might be expected, however, apparently for the reason that thetuned circuits were shock excited by the interference 4O currents toproduce relatively strong interfering currents at the signal frequency.

In accordance with the present invention, I have discovered thatrelatively simple filters can be constructed having sharp resonance inthe 45 upper audio range so that they pass currents of a singlefrequency or very narrow band of frequency within this range with goodefficiency while attenuating currents of other frequencies adjacentthereto. I have further found that such 50 filters are relatively immuneto shock excitation at this frequency. The particular filtersconstructed, however, also have the characteristic of resonatingstrongly at lower frequencies. In brief, they seem to resonate at afundamental 55 frequency relatively low in the audio range and at higherfrequencies which are near but usually not exactly at multiples orharmonics of the fundamental. In general, I have found that if twofilters of this type are constructed as nearly identical as possible,they will usually resonate at 5 substantially the same fundamentalfrequency but at slightly different higher frequencies. In accordancewith this invention, I impress the signal currents, which may becontaminated with interfering currents, equally on the input circuits oftwo such filters and connect the output circuits of the filters inopposition to a signal responsive device, and adjust the signal note tothe higher resonant frequency of one of the filters. The signal currentstherefore appear in the out- 5 put circuit of the one filter withgreater intensity than in the output circuit of the other filter; theydo not balance out, and therefore actuate the signal responsive device.However, because of the fact that the two filters have substantiallyuniform response at most frequencies, and particularly at theirfundamental resonance points, the interfering currents are largelybalanced. out.

Heretofore I have employed electro-mechanical filters having the abovementioned characteri tics because I have found them simple, easy andinexpensive to construct and successful in operation. However, it isquite possible that equally good or better results might be obtainedwith purely electrical filters having the same general attenuationcharacteristics. The invention in its broad aspects is not limited toany particular type of filter but rather to the particular combinationin a circuit of filters having certain over-all attenuationcharacteristics regardless of the internal construction of such filters.

Referring now to the drawings:

Fig. 1 is a schematic diagram of a complete C. W. radio telegraphreceiving system incorporating the invention;

Fig. 2 is a detailed view taken in the plane II-II of Fig. 1, showingdetails of construction of one of the electromagnetic units employed;

Fig. 3 is a graph illustrating the propagation characteristics of thefilters employed in the circuit of Fig. 1 at different frequencies; and

Fig. 4 is a graph illustrating the transfer efficiency of a balancingtransformer that may be employed in the circuit of Fig. 1.

Referring to Fig. 1, I have shown a heterodyne radio receiver I havingits input terminals connected between an antenna 2 and ground 3 andhaving an output line consisting of a pair of conductors 4 and 5 whichextend into a filter unit enclosed within the dotted line 6. The fil- 5tering unit has a pair of output conductors l and 8 which connectthrough a low frequency amplifier 9 to a head telephone 10.

Aside from the use of the filtering system enclosed within the dottedline 3, the circuit of Fig. l is in accordance with standard radiopractice, the heterodyne receiver l producing, in response to continuouswave telegraph signals impressed upon the antenna 2, trains of waves ofan audible frequency which are amplified in the low frequency amplifierQ and impressed upon the headphones lfi where they are converted intosound waves of corresponding frequency which appear as short and longimpulses, representing dots and dashes, respectively, of a singlemusical note corresponding to the frequency of the current appliedthereto. It is well known that by the use of a heterodyne receiver,which includes a local oscillator producing an oscillation that beatswith the received oscillations, the frequency of the audible noteproduced in the headphones it) may be varied from zero to frequenciesabove audibility and adjusted to a point most satisfactory to thelistener.

However, interfering currents, whether produced by natural static or bynearby electrical circuits. produce noise currents in a heterodynereceiver and these currents may be of sufiicient intensity in manyinstances to completely mask the desired signal currents. However, thesenoise currents differ in general from the signal currents in that theyhave no particular frequency characteristics but contain many componentsof difierent frequencies scattered throughout the audible range but ingeneral concentrated to a substantial extent in the lower portion of theaudio range. By inserting a filter of the construction next to bedescribed between the output circuit or" the receiver and theheadphones, I have found it possible to greatly attenuate the noisecurrents, which are heterogeneous in their frequency characteristics,from the signal note which may be of asingle desired frequency.

Referring again to Fig. 1, the filter enclosed within the dotted line 6comprises two filter units ii and 52, respectively, each of which unitsin turn consists of a pair of dynamic speaker elements l3 and id, and i5and i6, respectively. All of the units 53. M, 5 and E5 may be identicaland it will suffice to describe in detail one of them.

Thus the unit it consists of a field magnet having a central core H, theouter end of which forms one pole, and an annular portion l8 surroundingthe core ll and connected thereto at one end and defining an annularpole i9 juxtaposed to the end of the core 5? but spaced therefrom by asmall air 2'2. A field winding 2! is positioned about the core I? and isenergized by a. suitable source of direct current indicated as a batteryconnected to the winding 2! in series with a variable adjustingresistance Current flowing in winding 2! produces a strong magnetic fluxin the field magnet and an intense radial ma tic field with n the airgap 25.

Mount-e. I ap is a movable coil 26, which coil may be wound on acylindrical shell which supports it. The shell is in turn con nected toa supporting spider the outer edge of which is rig'dly at ached to thefield ma net. This construct .n is similar identical to that commonlyemployed n dyna akers. As shown in Fig. 2, the portion secured to thecylin an outer portion anchored to the magnets with radial portions 23interconnecting the central and outer portions. The purpose of thisspider mounting is to permit free longitudinal motion of the shell andcoil 24 while maintaining the shell and coil out of contact with thepole pieces of the field magnet.

The shell 25 of unit I3 is rigidly connected by a rod to the shell 25 ofunit l4 and likewise the shell 25 of unit I5 is rigidly connected by arod 3i to the shell 25 of the unit (5. To prevent any electromagneticcoupling between the units -'i. 15 and the units l4, 16, a shield 32 ofiron or other paramagnetic material is positioned between the units,suitable apertures being provided. in the shield 32 for the passage ofthe mechanical connecting rods 30 and 3!.

The movable coils 24 of the units I3 and 55 are connected in parallelacross the conductors 4 and 5 extending from the output terminals of thereceiver l. The moving coil of unit I4 is connected through a matchingtransformer 34 to a winding 35 of a balancing transformer 35 and themoving coil of unit I6 is connected through matching transformer 31 to aseparate winding 38 on the balancing transformer 36. Transformer 35 hasa third or tertiary winding 39 magnetically coupled to both the windings35 and 38 and elec rically connected to the conductors 1 and 8 leadingto the low frequency amplifier 9. The windings 35 and 38 can be shiftedwith respect to the tertiary winding 39 to vary the couplings betweeneach of windings 35 and 38 and the winding 39. By suitably positioningthe windings 35 and 38. they be made to induce substantially equal andop posite potentials in the tertiary winding 39 so that if equalcurrents are flowing in windings 3E and 38 they can be made to induceequal opposite potentials in winding 39, thereby neutralizing each otherand producing no resultant current in the winding 39.

As a result of the fact that the moving coil 24 of each filter unit issuspended in a strong magnetic field, mechanical forces tending tovibrate the coil will be produced if alternating current is impressed onthe moving coil and alternating currents will likewise be developed inthe moving coil if the latter is vibrated within its associated magneticfield by some external cause. Hence if alternating currents are appliedto the moving coil 24 of either unit l3 or unit 15, the moving coil andassociated shell will be caused to vibrate and the vibration transferredby the rod 30 or the rod 3| to the moving coil of unit M or l6, therebysetting up alternating currents in the moving coil of unit l4 or IE,which alternating currents are applied through the matching transformers34 and 31 to the associated winding of the balancing transformer 36.

As a result of the fact that the moving elements of each filter unit IIand i2 comprising the shells 25, coils 24, spiders 26 and the connectingrod 30 or 3|, as the case may be, have mass and resilience, they tend tovibrate with increased amplitude at certain resonant frequencies andwhen the windings 24 of the units (3 and i5 are energized withalternating currents of such frequencies the moving systems vibrate withincreased amplitude to develop currents of correspondingly increasedmagnitude in the coils 24 of the unit l4 and Hi.

In Fig. 3 the resonance characteristics of the fi ters H and. (2,respectively, are indicated by the curves 4!] and 4|, respectively, thecurve 4!! being shown in full lines and the curve 4| in dotted lines.The curves of Fig. 3 were prepared by applying alternating currents ofequal intensity but successively varying frequencies to the windings 24of the filter units l3 and I5, measuring the resultant potentialsdeveloped in the windings 24 of the filter units l4 and I6 and plottingthe latter voltages against frequency, the ordinates in Fig. 3representing the voltages developed in the windings 24 of units l4 andI6 and the abscissa representing the different frequencies.

It will be observed from Fig. 3 that each of the units H and I2 respondsstrongly at a frequency slightly less than 300 cycles and at frequenciesabove 1800 cycles. A slight peak is also noted at a frequency above 900cycles. It will also be observed that the peaks in curves 40 and 4| inthe neighborhood of 300 cycles occur at very nearly the same frequencybut that as the frequency increases the peaks in curves 40 occur atslightly higher frequencies than the peaks in curve 4|. This differenceis particularly noticeable in the peaks above 1800 cycles.

The four filter units l3, l4, l5 and I6 are all substantially identicaland differ from each other only as the result of minute unavoidablevariations in dimensions during manufacture. It appears that theseunavoidable variations in the dimensions of the parts account for thedifferences in the frequencies of maximum response in the region above1800 cycles. On the other hand, the differences in dimensions areinsufficient to produce any appreciable difference in the frequencies offundamental response below 900 cycles. The probable explanation for thisphenomenon is that the moving elements vibrate as a complete unit at thelower resonant frequency below- 900 cycles whereas at the higherfrequencies above 1800 cycles the resonance peaks result from secondaryor nodal vibrations in the spiders supporting the moving elements. Inother words, although two vibrating systems may have the samefundamental resonance frequencies, they will not have the same harmonicfrequencies unless their constants are uniformly distributed. Thevibrating systems of the units H and I2 may be analogous to vibratingstrings having weights thereon, the supporting spiders being theequivalent of the string and the coils 24 and the driving pins or rods30 and 3! constituting the equivalent of the weight on the string. Inthe case of two such strings having weights thereon, both might vibrateat the same fundamental frequency although the tension of one stringmight be less than the other and the weight on that stringcorrespondingly less than the weight on the other. However, such stringswould not have the same upper resonant frequencies. Considering furtherthe present invention, the spring constant of one set of spiders and theeffective weight of the coils and driving pins might both be less on oneunit than on the other, under which conditions the fundamental naturalperiods would be the same but the upper or harmonic periods different.

Whatever the specific explanation may be, I have found that inassembling two filter units II and if as shown in Fig. 1 from foursimilar units l3. l4, l5, and IS the two systems will in almost everycase have different frequencies of response in the upper range althoughthey may respond at substantially the same fundamental frequency.

Assume now that a continuous wave telegraph signal received on theantenna 2 along with static and/or miscellaneous interference fromvarious local sources. Assume further that the heterodyne receiver l isadjusted to deliver signal pulses of frequency corresponding exactly tothe peak in the curve 40 above 1800 cycles, this adjustment beingreadily effected by adjusting the frequency of the local oscillator inthe receiver I.

The signal impulses will be transmitted through the filter unit H withgreater efficiency than through the filter unit l2 by virtue of the factthat the signal occurs at exactly the upper resonant frequency of filterunit H whereas this frequency is substantially removed from thecorresponding resonant peak of the unit l2. Assuming that the windings35 and 38 of the balancing transformer 36 are adjusted to balance outequal currents therein, the stronger current produced in the winding 35in response to the signal will not be completely balanced out by thecurrent in the winding 38 and a resultant current will be induced in thetertiary winding 39 and applied through the low frequency amplifier 9 tothe receiver l0, producing an audible signal therein.

However, currents in the output of the receiver l resulting from staticor other interference will have no particular frequency characteristicand will contain components distributed over the entire audiblefrequency band but concentrated to a large extent in the lower portionof the band. These extraneous and undesired currents will in generalproduce strong currents in the windings 35 and 38 at the lower resonantfrequencies of the two filter units below 900 cycles but, since bothunits resonate at substantially the same frequency in this region, thecurrents of these frequencies produced in the windings 31 and 38 will besubstantially equal and will be balanced out so that they will appear toa very slight extent in the winding 39 and headphones l0.

Because of the fact that most of the undesired extraneous currentsappear in the lower portion of the audible frequency range, the systemcan be made even more selective for currents of the signal frequency byso designing the balancing transformer 36 and/or the matchingtransformers 34 and 31 as to cause them to transmit more efficiently inthe range of the signal frequencies than at lower frequencies. Thus asshown in Fig. 4 the transformers 34, 31 and 36 can be designed to have arising characteristic as indicated by the curve 42, thereby attenuatingthe currents in the lower audible range to a greater extent than thosein the vicinity of 1800 cycles.

It is to be understood that although in the system of Fig. 1 the signalsare shown applied through the low frequency amplifier 9 to a pair ofheadphones I 0, other types of signal responsive devices may beemployed. Thus a loud speaker might be substituted for the headphones IDor if desired these signals can be rectified and used to actuate relaysfor automatic recording. Such devices are well known in the art and donot constitute a part of the present invention.

It is not essential that the filter units II and I2 have equal averagetransmission efficiencies over the frequency range nor that thebalancing transformer 36 be adjusted exactly to neutral positionalthough it is possible to vary the transmission efficiencies of theunits H and I2 independently by varying the resistances 23 in the fieldmagnet circuits. As a matter of fact, the curves 4i; and Al representthe actual transmission efficiencies of two units II and I2 when theyare adjusted for most efficient operation with respect to theelimination of interference. It will be observed, however, that thecurve 40 is above the curve All substantially throughout the range. Theplausible explanation for this phenomenon is that the balancingtransformer 36 was not adjusted to exactly neutral position so that inorder to secure maximum suppression of interfering currents it wasnecessary to adjust the unit H to have a slightly higher transferefficiency at all frequencies than the unit 12.

It is not essential that the exact circuit arrangement shown in Fig. 1be employed in order to utilize the advantages of the invention. Thusinstead of connecting the moving coils 24 of units 10 i3 and in parallelacross the output of the receiver l, these coils may be connected ifdesired in series with each other and with the output circuit 2. Ofcourse, the impedances of the windings should be properly matched to theimpedance of the output circuit of the receiver l and it may often bedesirable to insert a suitable matching transformer or other network inthe output circuit of the receiver. Such expedients are well known inthe art.

The system as described has been found to be unusually immune to shockexcitation at the signal frequencies corresponding to the peaks above1869 cycles in the curves 40 and 4 Although this fact has beendefinitely established by experiment, the reason for such action is notfully .-1OWll to me at present. It may be that by virtue of therelatively great response of the filter elenents ii and 2 at thefundamental resonance uency below 900 cycles, the energy of the uniredcurrents of no particular frequency chareristics is largely absorbed bythe filter units des act

and converted into currents of this lower resonant frequency which arethen balanced out in the balancing transformer 35.

As previously indicated, many variations can in the particular circuitdisclosed, espewith reference to the specific construction e filterunits, without departing from the l-ntion and the latter is therefore tobe limited only to the extent set forth in the appended claims.

I claim:

A system of the type described for transting with relatively highefficiency electric 'es of a predetermined frequency while atheatingwaves of other frequencies, comprising an input line and an output line,a pair of filter e ements having input circuits connected ut line andhaving output circuits, ai1s for applying waves in the output circuits osaid filter elements to said output line in o posing relation wherebywaves of the same frequency and predetermined relative intensities 1 asoutput circuits of said filter elements neueach other and produce nowaves in the n but whereby waves of relative intensities other than saidpredetermined relative ensities in the output circuits of said filterelements produce a resultant wave in the output filter elements havinghigh transfer Lee at relatively low substantially idenre uencies andalso at relatively high disrequencies and the upper frequency of hightransfer efficiency of one of said filter elents being substantiallyidentical with said prenod frequency.

u of the type described for transwith relatively high efficiencyelectric cf predetermined frequency while atwaves of other frequencies,comprisinput line and an output line, a pair of filter elements havinginput circuits connected to said input line and having output circuits,11 cans for applying waves in the output circuits said filter elementsto said output line in opposing relation whereby waves of the samefrequency and predetermined relative intensities in the output circuitsof said filter elements neutralize each other and produce no waves inthe output line but whereby waves of relative intensities other thansaid predetermined relative intensities in the output circuits of saidfilter elements produce a resultant wave in the output line, said filterelements resonating at substantially identical fundamental frequencieslow in the audible range and also resonating at higher frequencies inthe audio range adjacent to multiple frequencies of the fundamentalfrequency, at least one of the upper resonating points of one filterelement being substantially removed in frequency from the nearestresonating point of the other filter element and the said upperfrequency at which one of said filters resonates being substantiallyidentical with said predetermined frequency.

3. A system of the type described for transmitting with relatively highefficiency electric waves of a predetermined frequency while attenuatingwaves of other frequencies, comprising an input line and an output line,a pair of iiter elements having input circuits connected to said inputline and having output circuits, means for applying waves in the outputcircuits of said filter elements to said output line in opposingrelation whereby waves of the same frequency and predetermined relativeintensities in the output circuits of said filter elements neutralizeeach other and produce no waves in the output iine but whereby waves ofrelative intensities other than said predetermined relative intensitiesin the output circuits of said filter elements produce a resultant wavein the output line; said filter elements comprising vibratoryelectrcdynainic devices including mechanical elements having naturalperiods of vibration with means responsive to electric waves applied tosaid input circuits for vibrating said mechanical elements, and meansresponsive to vibration of said mechanical elements for generatingelectric waves of corresponding frequencies in said output circuits, oneof said filter elements having a natural period of vibration at saidpredetermined frequency and the other filter element having acorresponding natural period of vibration displaced from saidpredetermined frequency.

4-. A system of the type described for transiiitting with relativelyhigh efficiency electric waves of a predetermined frequency whileattenuating waves of other frequencies, comprising input line and anoutput line, a pair of filter elements having input circuits connectedto said input line and having output circuits, means for applying wavesin the output circuits of said filter elements to said output line inopposing relation whereby waves of the same frequency and predeterminedrelative intensities in the o :tput circuits of said filter elementsneutralize each other and produce no waves in the output line butwhereby waves of relative intensities other than said predeterminedrelative intensities in the output circuits of said filter elementsproduce a resultant wave in the output line; filter elements comprisingvibratory electrodynamic devices including mechanical elements havingnatural periods of vibration with means responsive to electric waves an1 ed to said input circuits for vibrating said mechanical elements, andmeans responsive to vibration of said mechanical elements for generatingelectric waves of corresponding frequencies in said output circuits,said filter elements also including means for independently varying theamplitude of vibration of said mechanical vibratory elements in responseto electric waves of given amplitude in said input circuits and one ofsaid filter elements having a natural period of vibration at saidpredetermined frequency and the other filter element having a naturalperiod of vibration displaced from said predetermined frequency.

5. A system of the type described for transmitting with relatively higheificiency electric waves of a predetermined frequency while attenuatingwaves of other frequencies, comprising an input line and an output line,a pair of filter elements having input circuits connected to said inputline and having output circuits, means for applying waves in the outputcircuits of said filter elements to said output line in opposingrelation whereby waves of the same frequency and predetermined relativeintensities in the output circuits of said filter elements neutralizeeach other and produce no waves in the output line but whereby waves ofrelative intensities other than said predetermined relative intensitiesin the output circuits of said filter elements produce a resultant wavein the output line, in which said filter elements comprise vibratoryelectrodynamic devices including mechanical elements having naturalperiods of vibration with means responsive to electric waves applied tosaid input circuits for vibrating said mechanical elements, and meansresponsive to vibration of said mechanical elements for generatingelectric waves of corresponding frequencies. in said output circuits,said filter elements also including means for independently varying theamplitude of electric waves generated in said output circuits inresponse to vibration of given amplitudes in said mechanical vibratoryelements, and one of said filter elements having a natural period ofvibration at said predetermined frequency and the other having a naturalperiod of vibration displaced from said predetermined frequency.

6. A system of the type described for transmitting with relatively highefficiency electric waves of a predetermined frequency while attenuatingwaves of other frequencies, comprising an input line and an output line,a pair of filter elements having input circuits connected to said inputline and having output circuits, means for applying waves in the outputcircuits of said filter elements to said output line in opposingrelation whereby waves of the same frequency and predetermined relativeintensities in the output circuits of said filter elements neutralizeeach other and produce no waves in the output line but whereby waves ofrelative intensities other than said predetermined relative intensitiesin the output circuits of said filter elements produce a resultant wavein the output line, said filter elements having high transferefiiciencies at relatively low substantially identical frequencies andalso at relatively high dissimilar frequencies and the upper frequencyof high transfer efficiency of one of said filter elements beingsubstantially identical with said predetermined frequency, and means forvarying the relative coupling between said two output circuits of saidfilter elements and said output line.

'7. A system of the type described for transmitting with relatively highefiiciency electric waves of predetermined frequency in the upperportion of the audio range while attenuating waves of other frequencies,said system comprising an input line and an output line, a pair offilter elements having input circuits connected to said input line andhaving output circuits, means for applying Waves in the output circuitsof said filter elements to said output line in opposing relation wherebywaves of the same intensity and frequency in the output circuits of saidfilters neutralize each other and produce no waves in the output linebut whereas waves of the same frequency but unequal intensity produce aresultant wave of the same frequency in the output line, one of saidfilter elements being adapted to pass waves of said predeterminedfrequency more efiiciently than the other filter, said filter elementshaving at least approximately the same attenuation characteristics inthe lower portion of the audio range and having corresponding resonancepoints of substantial amplitude within said lower range, and said meansfor applying waves in the output circuits of said filter elements tosaid output line in opposing relation having a transfer efliciency whichincreases with frequency up to said predetermined frequency.

FREDERICK E. NICKEL.

